Retransmission Timeout Considerations

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Last updated 2015-11-23
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Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-allman-tcpm-rto-consider-03.txt            November 23, 2015
Intended Status: Best Current Practice
Expires: May 23, 2016

                 Retransmission Timeout Considerations

Status of this Memo

    This document may not be modified, and derivative works of it may
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    Copyright (c) 2015 IETF Trust and the persons identified as the
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    This document is subject to BCP 78 and the IETF Trust's Legal
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    ( in effect on the date of
    publication of this document. Please review these documents
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    Each implementation of a retransmission timeout mechanism must
    balance correctness and timeliness and therefore no implementation
    is suits all situations.  This document provides for high-level
    guidance for retransmission timeout schemes appropriate for general

Expires: May 23, 2016                                           [Page 1]
draft-allman-tcpm-rto-consider-03.txt                      November 2015

    use in the Internet.  Within the guidelines, implementations have
    latitude to define particulars that best address each situation.


    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in BCP 14, RFC 2119

1   Introduction

    Despite our best intentions and most robust mechanisms, reliability
    in networking ultimately requires a timeout and re-try mechanism.
    Often there are more timely and precise mechanisms for repairing
    loss (e.g., TCP's fast retransmit [RFC5681], NewReno [RFC6582] or
    selective acknowledgment scheme [RFC2018,RFC6675]) which require
    information exchange between components in the system.  Such
    communication cannot be guaranteed.  Alternatively, information
    coding can allow the recipient to recover from some amount of lost
    information without use of a retransmission.  This latter provides
    probabilistic reliability.  Finally, negative acknowledgment schemes
    exist that do not depend on positive feedback to trigger
    retransmissions (e.g., [RFC3940]).  However, regardless of these
    useful alternatives, the only thing we can truly depend on is the
    passage of time and therefore our ultimate backstop to ensuring
    reliability is a timeout.  (Note: There is a case when we cannot
    count on the passage of time, but in this case we believe repairing
    loss will be a moot point and hence we do not further consider this
    case in this document.)

    Various protocols have defined their own timeout mechanisms (e.g.,
    TCP [RFC6298], SCTP [RFC4960]).  Ideally, if we know a segment will
    be lost before reaching the destination, a second copy of it would
    be sent immediately after the first transmission.  However, in
    reality the specifics of retransmission
    timeouts often represent a particular tradeoff between correctness
    and responsiveness [AP99].  In other words we want to

      - Wait long enough to ensure the decision to retransmit is

      - Bound the delay we impose on applications before
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